For engine systems in vehicular or other mobile applications where a supply of hydrogen is utilized, due to challenges related to on-board storage of a secondary fuel and the current absence of a hydrogen refueling infrastructure, hydrogen is preferably generated on-board using a fuel processor. The hydrogen-containing gas from the fuel processor can be used to regenerate, desulfate and/or heat engine exhaust after-treatment devices, can be used as a supplemental fuel for the engine, and/or can be used as a fuel for a secondary power source, for example, a fuel cell. In some applications the demand for the hydrogen-containing gas produced by the fuel processor is highly variable.
One type of fuel processor is a syngas generator (SGG) that can convert a fuel reactant into a gas stream containing hydrogen (H2) and carbon monoxide (CO), known as syngas. Air and/or a portion of the engine exhaust stream can be used as an oxidant reactant for the fuel conversion process. Steam and/or water can optionally be added. The SGG can be conveniently supplied with a fuel comprising the same fuel that is used to operate the engine. Alternatively a different fuel can be used, although this would generally involve a separate secondary fuel source and supply system specifically for the SGG. The syngas can be beneficial in processes used to regenerate exhaust after-treatment devices. For other applications, for example, use as a fuel in a fuel cell, the syngas stream can be additionally processed prior to use.
The thermochemical conversion of a hydrocarbon fuel to syngas is performed in an SGG at high operating temperatures with or without the presence of a suitable catalyst. Parameters including equivalence ratio (ER) and operating (reaction) temperature are typically adjusted in an attempt to increase the efficiency of the fuel conversion process while reducing the generally undesirable formation of carbon (coke or soot) and other deposits, which can cause undesirable effects within the SGG and/or in downstream components.
The term equivalence ratio (ER) herein refers to a ratio between the actual amount of oxygen supplied and the theoretical stoichiometric amount of oxygen which would be required for complete combustion of the fuel. An ER of greater than 1 represents a fuel lean mode (excess oxygen), while an ER of less than 1 represents a fuel rich mode (excess fuel). The term carbon herein includes solid fraction particulates of carbon including amorphous carbon, coke and soot, as well as carbonaceous gums, resins and other deposits.
Over time, carbon accumulation can impede the flow of gases, increase the pressure drop across the SGG and its associated components, and reduce the operating life or durability of the SGG. Large accumulations of carbon also have the potential to create excessive amounts of heat that can damage the SGG if the carbon is converted (for example, combusted, oxidized or gasified) in an uncontrolled manner, for example, in a short period of time.
While many have attempted to eliminate or reduce carbon formation, practically there is an inevitable tendency for carbon to form during the conversion of the fuel into syngas. A particulate filter, also known as a particulate trap, soot filter or soot trap, can be employed at least partially within or downstream of a fuel processor to collect or trap carbon. This allows for increased control and management of the particulates. The particulate filter can be, for example, a wall-flow monolith, a fibrous structure, a foam structure, a mesh structure, an expanded metal type structure or a sintered metal type structure. The particulate filter can be constructed from a suitable material, for example, ceramic materials, and can optionally contain one or more catalysts. Typically, carbon can be allowed to collect until the accumulation begins to adversely affect the gas flow across the particulate filter. A subsequent carbon removal or conversion process can be initiated to remove the carbon particulates collected by the particulate filter, then it will continue to trap carbon particulates. The term “carbon gasification” herein includes one or a combination of oxidation or other carbon conversion processes by which carbon is reacted to form syngas.
In certain applications an SGG can be operated in a so-called “lean-rich” regime, in which the SGG is cycled between operating in a lean mode and operating in a rich mode. This is typically done for carbon removal purposes and/or temperature control purposes and/or to accommodate intermittent syngas demand. During first periods of time, the SGG is operated at a substantially less than stoichiometric ER (or “rich” mode) to produce a syngas stream, which can also result in the formation of carbon. During intervening second periods of time, the SGG is operated at a substantially greater than stoichiometric ER (or “lean” mode) to combust accumulated carbon. A drawback of such a method is that the production of syngas ceases when the SGG is operated in the lean mode, so it is less suitable in situations where the demand for syngas is prolonged or continuous.